During the manufacture of metal cans, container bodies are inspected and tested for openings in the bottom and side wall surfaces or seams prior to completion of the fabrication process. Such defects may occur in the side or bottom areas or at the top flange areas of the containers.
Conventionally, an apparatus capable of handling a large number of can bodies transfers them at high speed through an inspection area in which defects are found. FIG. 1A depicts the conveyance path of the containers. From the infeed area 10, container bodies are advanced to be detected at station 20. The bodies are further conveyed to a discharge area 30 for either rejection of faulty containers or for advancement for further processing. Conventional apparatuses of this nature are described in U.S. Pat. No. 3,750,877 to Cvacho et al. and U.S. Pat. No. 4,305,816 to Flood et al. Cans are inserted in a continuously rotating carrier, which cooperates with a conveyance mechanism, for transfer to the inspection area. Ejection of the cans after inspection to a rejection chute is effected by a vacuum actuator if a fault determination has been made.
The inspection is performed by a photoelectric device, including a photosensor, mounted at a fixed, stationary location on the apparatus. A circular opening in the device permits light to be sensed. Cans are rotatively moved through a housing wherein the external surfaces of the cans are illuminated. The cans are uniformly positioned in the carrier at a given radial distance from the axis of rotation and oriented horizontally with their axes in parallel with the carrier axis of rotation. An opaque disk, containing a plurality of circular windows, is mounted on the carrier for rotation therewith. The windows are smaller in diameter than the cans to be inspected. During rotation of the carrier, cans are successively urged to be in contact with the disk. The open top portion of each can is aligned with, and surrounds, a window of the disk.
At the inspection area the opaque disk, positioned between the exteriorly illuminated cans and the photoelectric device, shields the latter from unwanted light. As the carrier rotates, the cans, aligned with the disk windows, successively come into registration with the opening in the photoelectric device. The disk windows serve as viewing ports for discerning passage of light through openings in the can bodies.
FIG. 1B illustrates the prior art relationship at the inspection area. The disk 1, shown in broken section, rotates in the clockwise direction about axis 2 as represented by the arrow. Windows 3 in the disk are evenly distributed along the disk at a radial distance 4. Cans, held in pockets (not shown) in the carrier and in contact with the disk, are shown having circumferences 5 which are concentric with the plate disk windows 3. Light seals, indicated at 7, are provided between pockets. The window 3 serves as a can viewing port for the opening 6 of a single stationary light sensor device. This device is responsive to light transmitted from the light source through any opening in a faulty can and the viewing port as the can passes through the optical path. In such case a signal is produced causing actuation of a compressed air device (not shown) to eject the can into a defect chute (also not shown) at the appropriate point in the travel path.
In the above described prior art apparatus the speed at which the cans can be processed is limited by the capability of the single sensor to respond to each inspection. For example, a typical arrangement would provide cans spaced at 30.degree. increments on an 18 inch diameter circle. The cans would then be spaced 4.7 inches apart with a viewing port area of approximately 3 square inches. At a feed rate of 2000 cans per minute, the light sensor has 7.6 mil. sec. to view the inside of the can. This rate taxes the reliability of the sensor. While the inspection rate can be increased by reducing the spacing between cans, there must be room enough to permit illumination of all exterior surfaces.
A drawback of the conventional device is that it cannot distinguish the location of a fault. The detector is responsive to light from wherever the source. It would be useful to determine, for example, whether there are consistent faults in the container top flange area, as opposed to occurrences of random holes in the sheet material or fractures at the bottom; such information would indicate the need for investigation of the fabrication process.